The main objective of the proposed research is to study the organization of the sensory thalamus and cortex. The afferent and efferent connections of two thalamic regions, the audiotry and the visual, will be described using the methods of anterograde degeneration and autoradiography. The primary gaols of this phase of the experiments are first, to determine how many separate pathways to cortex exist for each sensory system and then, to describe the connectional relationship between these pathways and the cortex. A second phase of these experiments is designed to compare the synaptic arrangements in two thalamic visual relays, the pulvinar nucleus and the lateral geniculate using electron microscopic techniques. A number of different species including iguanas, hedgehogs, squirrels, tree shrews and monkeys will be studied in an effort to determine how these thalamic and cortical areas vary among species.